Choking smog all but shut down one of northeastern China's largest cities on Monday, forcing schools to suspended classes, snarling traffic and closing the airport, in the country's first major air pollution crisis of the winter.

An index measuring PM2.5, or particulate matter with a diameter of 2.5 micrometers (PM2.5), reached a reading of 1,000 in some parts of Harbin, the gritty capital of northeastern Heilongjiang province and home to some 11 million people.

A level above 300 is considered hazardous, while the World Health Organisation recommends a daily level of no more than 20.

Smog is the brownish haze that pollutes our air, particularly over cities in the summer. Smog can make it difficult for some people to breathe and it greatly reduces how far we can see through the air.

The primary component of smog is ozone, a gas that is created when nitrogen oxides (NOx) react with other chemicals in the atmosphere, especially in strong sunlight. NOx is produced whenever we burn something, such as coal in a power plant or gasoline in a car's engine.

New Delhi might be knocking at its door but Beijing is not ready to give up its pollution-related dubious distinction without a fight.

Unfavourable weather and heavy fireworks during last week's Lantern Festival have pushed up pollution levels in Beijing to dangerous levels with the government coming under sharp criticism for not having declared the necessary alerts.

Beijing residents have woken up to dark, smoggy skies effectively blanketing the sprawling city for three days in a row now.

On Sunday morning, Sunday, the air quality index (AQI) at monitoring stations in the city's downtown areas read between 424 and 470 at Level 6, the highest level indicating hazardous pollution, according to the Beijing Municipal Environmental Monitoring Center's website.

The major pollutants were PM 2.5 and PM 10, the ministry of environment protection was quoted by the state media.

As many 33 of the 161 cities monitored nationwide were hit by heavy air pollution.

"Beijing, together with its surrounding cities such as Shijiazhuang, Baoding, Xingtai and Hengshui, was rated by the Ministry of Environmental Protection as 'severely polluted' on Friday," according to the ministry.

The level of pollution has even forced the state media to criticise government inaction.

"Beijing municipal government, don't pretend to be blind taking advantages of the fog," national broadcaster, China Central Television's (CCTV)'s business channel said on its official microblog website at the Twitter-like weibo.com.

"The government should not shun its responsibility or turn a blind eye to the smog," it said.

Communist Party of China mouthpiece, the People's Daily online said the city has a four-tier alert system, using blue, yellow, orange and red to indicate the lowest to the highest level of air pollution.

Last October, following weeks of heavily polluted and smoggy days on more than one occasion, the Beijing Municipal Government had put in place an emergency response system.

"The system requires that traffic be cut with alternate driving days for even- and odd-numbered license plates and schools be suspended if a red alert, the highest level for air pollution, is issued. Industrial plants will be closed or told to reduce production if an orange alert, the second highest one, is issued," the online report said.

However, the report added that since the regulations were put in place, the government has not initiated the emergency response once, although the public for several times believed the smog was heavy enough for a government response," Communist Party of China mouthpiece, People's Daily said.